Lithography is used in the fabrication of semiconductor devices. It is the technique by which the patterns that make up the circuitry of a chip are defined. In lithography, a light-sensitive material coats a wafer substrate, such as silicon, that is exposed to light at some wavelength to reproduce an image of the mask that is used to define each die on the wafer. The mask is usually referred to as a reticle in optical lithography, in that it is separated from the wafer by a series of objective lenses. When the wafer and mask are illuminated, the light-sensitive material, or photoresist, undergoes chemical reactions to produce a replicated pattern of the mask on the wafer. The patterns can then be manipulated in ways to transform the structure or properties of the wafer, which leads to the creation of various semiconductor devices and applications.
The source of the light used in lithography, for example, can be an excimer laser light sources at wavelengths of 248 nm, 193 nm or 157 nm. Extreme Ultraviolet (EUV) light sources, which produce light with a wavelength of approximately 13 nm, can also be used.
EUV light can be produced using a small, hot plasma which will efficiently radiate at a desired wavelength, for example 13.4 nm. The plasma is created in a vacuum chamber, typically by driving a pulsed electrical discharge through the target material, or by focusing a pulsed laser beam onto the target material. The light produced by the plasma is then collected by nearby mirrors and sent downstream to the rest of the lithography tool.
The hot plasma tends to erode any materials nearby, for example the electrodes in electric-discharge sources, or components of the gas delivery system in laser-produced plasmas. The eroded material may then coat the collector optics, resulting in a loss of reflectivity and reducing the amount of light available for lithography.
The collector optics may be replaced once they are coated with a given level of debris. Alternatively, the collector optics could be cleaned, for example, by temporarily removing them from the system for cleaning. The optical components may then be put back in the EUV system, and recalibrated and realigned.